IM13

The existence of global time in the HDL side gives a natural order to

two messages which occur at different uclocks. So if message A occurs

at uclock u(A) and message B occurs at u(B) then A happens before B if

u(A) < u(B). That order was respected for message delivery. This

allows synchronous or reactive models as well as asynchronous or

streaming ones to be written easily. In the case that u(A)==u(B) there

is some ambiguity in ordering. This is where the priority mechanism

came in.

Especially with reactive models, which could implement state machines
based on the messages they receive, the global time of HDL must be
respected. If implementations were allowed to ignore the global time
in HDL, synchronous and reactive models would become far too difficult
to implement.

Since the software side has no notion of global time other than that
inherited through its interaction with HDL, the requirements for
ordering of input messages are different. The user is responsible for
guaranteeing one of three conditions:

             Input Message A happens before input message B

             Input Message B happens before input message A

             the order of input messages A and B is irrelevant.

Duaine

 
Received on Wed Mar 24 12:03:58 2004